Someone figured out a way to get their computer into the hardware of the system and decided to broadcast some porn during rush hour (you know, real F-Society-"V For Vendetta"-"Black Mirror" kind of stuff).

Or...

2) Some dude forgot to log out of his work computer, went home and decided to make love to himself. Because he didn't log out, the image was still being transmitted over the billboard's airwaves and everyone got a little X-rated side show on their way to work.